Prosthetic joint

ABSTRACT

A prosthetic knee joint is provided having an extended position, an intermediate position, and a flexed position. The motion of the joint includes a minor segment from the extended position to the intermediate position, and a major segment from the intermediate position to the flexed position. The center of pressure between the femoral component and the tibial component moves rearward on the tibia during the minor segment. During the major segment, the joint flexes about an axis of rotation with the bearing surfaces on the femoral and tibial components being in congruent engagement. The distal surface of the femoral component includes two rails for engagement with a patellar prosthesis. The contour of the rails is either a straight line or a concave curve to provide line contact between the rails and the patellar prosthesis. In certain embodiments, the patellar prosthesis has a saddle-shaped surface so that the prosthesis and each of the rails can make contact over an area extending along the length of the rail.

This is a continuation of co-pending application Ser. No. 07/151,429,filed on Feb. 2, 1988 now U.S. Pat. No. 4,888,021.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved prosthetic joints and in particularto improved prosthetic knee joints.

2. Description of the Prior Art

Flexion and extension of the normal human knee involves complexmovements of three bones: the femur, the tibia, and the patella. Duringflexion, the distal end of the femur and the proximal end of the tibiarotate and glide relative to one another, with the center of rotation ofthe joint moving posteriorly over the condyles of the femur; duringextension, the tibia and femur follow the reverse path, with the centerof rotation now moving anteriorly as the joint is extended. Simultaneouswith these movements of the femur and tibia, the patella moves over thesurface of the femoral condyles, while remaining at a relativelyconstant distance from the tubercle of the tibia through the attachmentof the patella to the tibia by the patellar ligament.

Numerous prostheses have been proposed as replacements for the naturalknee joint. See, for example, Noiles, U.S. Pat. Nos. 3,996,624,4,219,893, and 4,301,553, Averill, U.S. Pat. Nos. 3,728,742 and4,217,666, Insall, U.S. Pat. No. 4,213,209, Tavernetti, U.S. Pat. No.3,813,700, German Patent Publications Nos. 2,227,090 and 2,501,128, andFrench Patent Publications Nos. 2,269,324 and 2,478,462. For total kneereplacements, the condyles of the femur and the head of the tibia aresurgically removed and replaced with prosthesis components. A patellarprosthesis, e.g., a spherically-domed or conical plastic button, isnormally attached to the posterior surface of the patella to serve as aninterface between the patella bone and the femoral prosthesis.

Efforts have been made to produce prosthetic joints which function in amanner similar to the natural knee. Specifically, a number of mechanismshave been proposed for producing posterior movement of the femoralcomponent relative to the tibial component (femoral roll-back on thetibia) as the joint is flexed. For example, Walker et al., U.S. Pat. No.4,209,861, discloses a prosthetic knee joint wherein guiding surfaces onthe femoral and tibial components are used to induce posterior movementon the tibial component of the contact area between the components asthe knee is progressively flexed. The posterior movement takes placethrough a major portion of the flexion of the joint. Burstein et al.,U.S. Pat. No. 4,298,992, shows an alternate construction in which thefemoral component moves posteriorly relative to the tibial component ator near full flexion. See also Deane, U.S. Pat. No. 3,840,905.

These prior art constructions suffer the common disadvantage that thefemoral and tibial bearing surfaces are only in contact over smallareas. Moreover, the contact areas become even smaller when the joint isflexed. During flexion, e.g., during such activities as squatting, stairclimbing, or rising from a chair, high loads are applied to the jointand must be carried by the contact area between the bearing surfaces.Small contact areas plus high loads lead to high rates of wear of thebearing surfaces, which is clearly undesirable. U.S. Pat. No. 4,634,444to Douglas G. Noiles discloses a knee joint having bearing surfaces oflarge areas. However, the femoral component of this joint does not moveposteriorly relative to the tibial component during flexion, as occursin the natural knee.

Efforts have also been made to improve the functioning of patellarprostheses. See, for example Pappas et al., U.S. Pat. No. 4,470,158,Buechel et al., U.S. Pat. No. 4,309,778, and Buechel et al., U.S. Pat.No. 4,340,978. In particular, the anterior surfaces of femoralcomponents have been provided with concave recesses for receivingpatellar prostheses when the joint is at or near its fully extendedposition. See, for example, Forte et al., U.S. Pat. No. 4,353,135, andWalker, U.S. Pat. No. 4,209,861. Similarly, the distal surfaces offemoral components have included tracks for receiving the patellarprosthesis when the joint is flexed. Significantly, the surfaces whichengage the patellar prosthesis on these prior art tracks have beenconvexly shaped. Indeed, discontinuities in the slope of the prosthesis'outer surface have existed at the intersection between the concaverecess of the anterior surface and the convex track of the distalsurface.

Prostheses employing convex tracks have suffered a number ofdisadvantages. When used with the typical spherically-domed or conicalpatellar prosthesis, the track and the patellar prosthesis have onlymade point contact. As discussed above, prosthetic knee joints aresubject to high loads when flexed, i.e., when the patellar prosthesis isin contact with the distal track. This combination of high loads andpoint contact has resulted in high wear rates for the patellarprosthesis. Indeed, for patellar prostheses consisting of a plasticbearing mounted on a metal backing plate, complete wear through of thebearing so as to cause the metal plate and the metal femoral componentto grind against one another in situ, has been observed.

In addition to the point contact problem, the discontinuity in the outersurface of the femoral prosthesis at the intersection between theconcave recess and the convex track has also contributed to wearing ofthe patellar prosthesis and has degraded the overall smooth operation ofthe prosthesis.

The Forte et al. patent referred to above discloses a construction for apatellar prosthesis which can achieve line contact with a convex track.This construction, however, employs a complex patellar button geometrywhich must be precisely aligned with the femoral prosthesis during thesurgical procedure for the system to operate properly. Also, in revisionsurgery, the existing patellar prosthesis is normally not replaced. Mostexisting patellar prostheses are of the conical or spherically-domedbutton type. The Forte et al. construction, like the rest of the priorart constructions, only provides point contact when used with suchspherically-domed or conical patellar prostheses.

SUMMARY OF THE INVENTION

In view of the foregoing state of the art, it is an object of thepresent invention to provide improved prosthetic joints and inparticular improved prosthetic knee joints.

More particularly, one of the objects of the invention is to provide aprosthetic joint composed of two components, e.g., a femoral componentand a tibial component, wherein the first component translates relativeto the second component as the joint moves from its extended to itsflexed position and wherein the area of contact between the bearingsurfaces of the first and second components is large, and, inparticular, is large when the joint is flexed. Another object of theinvention, is to provide a femoral prosthesis whose distal surface isconfigured to provide line contact, as opposed to point contact, with apatellar prosthesis, including line contact with the spherically-domedand conical button-type prostheses which are typically encountered inrevision surgery.

To achieve the foregoing and other objects, the invention in accordancewith certain of its aspects provides an artificial joint which has anextended position, an intermediate position, and a flexed position. Themotion of the joint includes a minor segment and a major segment, theminor segment comprising movement between the extended and theintermediate positions, and the major segment comprising movementbetween the intermediate and the flexed positions.

The joint comprises two components, e.g., for a knee prosthesis, thefemoral component and the tibial component. Each component includes abearing surface and a cam member. Each bearing surface includes a firstportion and a second portion, the first portions being in engagementwith one another during the minor segment of the joint's motion, and thesecond portions being in engagement with one another during the majorsegment of the joint's motion. In certain preferred embodiments, thesecond portions comprise large area, stepped bearing surfaces of thetype disclosed in U.S. Pat. No. 4,634,444, referred to above, thepertinent portions of which are incorporated herein by reference.

During the major segment of the joint's motion, the motion of the jointcomprises rotation about an axis of flexion. The rotation takes placethrough sliding of the bearing surfaces of the second portions relativeto one another. The cam members on the first and second componentsinteract with one another during the minor segment of the joint'smotion. In particular, the cam members permit the first portions of thebearing surfaces to roll relative to one another as the joint movesbetween its extended and intermediate positions.

In a preferred construction of the joint, the area of contact betweenthe second portions of the bearing surfaces is greater than the area ofcontact between the first portions. This construction maximizes theoperational bearing contact area of the joint since, as discussed above,the second bearing surfaces are in contact during the major segment ofthe joint's motion. Moreover, for knee joints, this constructionprovides a large area of contact during flexion of the joint, i.e.,during times when the joint is subject to high loading forces.

In accordance with other aspects of the invention, a femoral prosthesisis provided whose outer surface has an anterior portion and a distalportion. The anterior portion includes a recess for engagement with apatellar prosthesis, and the distal portion includes a track for thesame purpose. The track is composed of two rails which intersect therecess. The surface of each of the rails has a cross-sectional contourin a direction transverse to the longitudinal axis of the rail which is(1) either a straight line or a concave curve, (2) constant along thelength of the rail, and (3) matches the contour of the surface of therecess at the intersection between the rail and the recess.

By means of this structure, the patellar prosthesis moves smoothly overboth the anterior and distal surfaces of the femoral prosthesis,including the transition between those surfaces. Moreover, the patellarprosthesis makes line contact with the distal surface of the femoralprosthesis. In particular, when the contour of the rails is a portion ofa straight line, line contact with conical patellar buttons is achieved,and when the contour of the rails is a portion of a concave circle, linecontact with spherically-domed patellar buttons is achieved.

The accompanying drawings, which are incorporated in and constitute partof the specification, illustrate the preferred embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention. It is to be understood, of course, thatboth the drawings and the description are explanatory only and are notrestrictive of the invention. In particular, it is to be understood thatalthough, for ease of discussion, the description which appears below isin terms of an artificial knee joint, various aspects of the inventionare equally applicable to other types of artificial joints, such as,artificial elbow joints and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, exploded view of a semi-constrained artificialknee joint constructed in accordance with the present invention.

FIG. 2 is a side view of the tibial plateau component of the joint ofFIG. 1.

FIG. 3 is a view of the anterior surface of the femoral component of thejoint of FIG. 1 showing the engagement of the patellar component of thejoint of FIG. 1 with the anterior surface.

FIG. 4 is a view of the anterior surface of the femoral component of thejoint of FIG. 1 showing the engagement of the patellar component withthe distal surface of the femoral component.

FIG. 5 is a view of the distal surface of the femoral component of thejoint of FIG. 1 showing the engagement of the patellar component withthe anterior surface.

FIG. 6 is a cross-sectional view along lines 6--6 in FIG. 4.

FIGS. 7-10 are side views of the joint of FIG. 1 at flexion angles of0°, 16°, 45°, and 120°, respectively.

FIG. 11 is a side view of the joint of FIG. 1 at a hyperextended angleof -6°.

FIGS. 12 and 13 compare the engagement between the patellar and femoralcomponents achieved with the present invention (FIG. 13) with thatachieved with prior art prostheses (FIG. 12).

FIG. 14 is a perspective, exploded view of a constrained artificial kneejoint employing the patella tracking system of the present invention.

FIG. 15 is a cross-sectional view of the joint of FIG. 14 along themidline of the prosthesis.

FIG. 16 is a side view of the femoral and patellar components of thejoint of FIG. 14.

FIG. 17 is a view of the distal surface of the femoral component of thejoint of FIG. 14 showing the engagement of the patellar component withthe distal surface.

FIG. 18 is a perspective view of a patellar prosthesis having asaddle-shaped surface.

FIGS. 19 and 20 are sectional views in sagittal planes comparing theengagement of the femoral component of the joint of FIG. 14 with aspherically-domed patellar button (FIG. 20) and with the patellarprosthesis of FIG. 18 (FIG. 19).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures, there is shown in FIG. 1 an exploded viewof a semi-constrained artificial knee joint constructed in accordancewith the present invention. The joint includes a femoral component 10and a tibial component 13 comprising tibial plateau component 12 andtibial sleeve component 14. As discussed in detail below, the joint isdesigned to smoothly interact with patellar prosthesis 80.

Femoral component 10 and tibial plateau component 12 respectively carrymating convex bearing surface 20 and concave bearing surface 34 (seeFIG. 1). As shown in FIG. 6, femoral convex bearing surface 20 iscomposed of part 20a described by radius R₁, part 20b described byradius R₂, and part 20c described by radius R₃. Femoral convex surface20 also includes part 20d described by radius R₄. Radius R₄ has the samecenter as radius R₃, and therefore surface 20d is concentric withsurface 20c. Part 20b is also referred to herein as the first portion ofconvex bearing surface 20; the combination of parts 20c and 20d are alsoreferred to herein as the second portion of convex bearing surface 20.

As shown in FIG. 2, tibial concave bearing surface 34 is composed ofpart 34a described by radius R₁, part 34b which may be flat or concavewith a radius greater than radius R₂ of femoral component 10, and part34c described by radius R₃. Tibial concave surface 34 also includes part34d described by radius R₄. Radius R₄ has the same center as radius R₃,and therefore surface 34d is concentric with surface 34c. Part 34b isalso referred to herein as the first portion of concave bearing surface34; the combination of parts 34c and 34d are also referred to herein asthe second portion of concave bearing surface 34.

As shown in FIGS. 1 and 5, each of parts 20a, 20b, and 20c, and parts34a, 34b, and 34c are composed of two spaced-apart sections. Thespaced-apart sections of parts 20c and 34c, in combination with parts20d and 34d, respectively, form stepped bearings of the type disclosedin U.S. Pat. No. 4,634,444, referred to above. As shown in the figures,these stepped bearings extend across the full width of the prosthesis soas to provide a large, wear-resistant bearing surface for flexionmotions of the joint. Preferably, parts 20c and 20d, i.e., the secondportion of convex bearing surface 20, and parts 34c and 34d, i.e., thesecond portion of concave bearing surface 34, are surfaces ofrevolution, i.e., cylindrical in shape, although other bearing contourscan be used in the practice of the invention.

So that the bearing surfaces can come apart in a direction orthogonal totheir axis of rotation, the second portion of concave bearing surface 34encompasses less than one-half of the second portion of convex bearingsurface 20. In particular, as shown in FIG. 9, the second portionsengage each other over an angle A, which for the embodiment shown isapproximately 15°.

The spaced-apart sections of part 34c are connected to part 34d by walls44. Similarly, the spaced-apart section of part 20c are connected topart 20d by walls 28. The presence of these walls stabilizes theassembled joint against dislocations along the axis of rotation of thesecond portions of bearings 20 and 34. Specifically, the engagement ofthe walls limits the lateral motion of surfaces 20 and 34 with respectto one another. Significantly, this stabilization is achieved withoutsacrificing the overall width of the bearing surfaces, as would occurwith other modes of lateral stabilization known in the art, such as,through the use of a post or the like between two laterally separatedbearing surfaces.

As shown in the figures, the outer sections of parts 20c and 34c haveequal radii of curvature, and those radii of curvature are larger thanthe radius of curvature of parts 20d and 34d. It is to be understoodthat the bearing surfaces can have radii of curvature other than thoseshown, provided that the radii are such that their differences producewalls 28 and 44 of sufficient height to restrain the joint againstlateral dislocations.

As can best be seen in FIG. 2, in addition to bearing surfaces 34,tibial plateau component 12 also includes cam means 41. Similarly, ascan be seen in FIG. 6, femoral component 10 includes cam means 43. Cammeans 41 is located between the spaced-apart sections of part 34b andcomprises an extension of part 34d. Cam means 43 is located between thespaced-apart sections of part 20b and comprises an extension of part20d. Cam means 41 is connected to part 34b by extensions of walls 44.Similarly, cam means 43 is connected to part 20b by extensions of walls28. These extensions can also engage against one another to helprestrain the joint at flexion angles against lateral dislocations.

The operation of cam means 41 and 43 is illustrated in FIGS. 7-10, whereFIG. 7 shows the joint in its extended position, FIG. 8 shows the jointin its intermediate position, and FIGS. 9 and 10 show the joint atflexion angles of 45° and 120°, respectively.

As shown in these figures, parts 20b and 34b, i.e., the first portionsof surfaces 20 and 34, are in engagement in the joint's extendedposition (FIG. 7) and roll relative to one another as the joint movesfrom its extended position to its intermediate position (FIG. 8). Thetheoretical contact between parts 20b and 34b during this rolling isline contact. Cam means 41 and 43 interact during this portion of thejoint's motion to allow and control the rolling between the bearingsurfaces. The natural forces in the knee tend to keep the cam means incontact through the minor segment of the joint's motion.

Parts 20c and 34c, as well as parts 20d and 34d, come into engagement atthe intermediate position and remain in engagement throughout theremainder of the flexion of the joint (FIGS. 9-10). The motion of thejoint when these second portions of surfaces 20 and 34 are in engagementconsists of simple rotation of the joint about the axis defined by thecommon center of radii R₃ and R₄. The second portions slide on oneanother during this rotation.

The transition from the engagement of the first portions, which, asdiscussed above, is theoretically just line contact, to the engagementof the second portions results in an increase in the contact areabetween the bearing surfaces. Specifically, the contact area increasesbecause of the congruent meshing of part 20c with part 34c and part 20dwith part 34d. Flexing beyond the intermediate position is accomplishedwith substantial bearing areas in contact to resist the high femur totibia loads created by weight bearing at greater flexion angles. For thejoint of FIG. 1, this congruent bearing area is on the order of 1.0square inch.

To maximize the contact area between the bearing surfaces through themajor segment of the motion of the joint, the transition between thefirst and second portions of the bearing surfaces is performed early inthe flexion of the joint. In FIGS. 7-10, the transition takes place at aflexion angle of about 16° from the extended position of the joint. Themotion of the joint thus consists of a minor segment from 0° to about16°, and a major segment from about 16° to the flexed position of thejoint, e.g., 100° to 120°, with the major segment being about 5 timesgreater than the minor segment.

The transition point between the first and second portions of thebearing surfaces can of course be set at flexion angles either greaterthan or less than 16°. In general, the transition point should occur ata flexion angle of less than about 30° in order to obtain the fullbenefits of the enhanced bearing surface contact area provided by theengagement of the second portions.

In addition to moving between its extended and flexed positions, thejoint of FIG. 1 can also be hyperextended. The amount of hyperextensionpermitted is determined by the engagement of surface 72 on femoralcomponent 10 (see FIG. 6) with surface 70 on tibial plateau 12 (see FIG.2). Radius R₁ of femoral surface 20a also comes into contact with radiusR₁ of tibial surface 34a which further inhibits hyperextension. FIG. 11shows the joint in its fully hyperextended condition. For the jointshown, the hyperextension is limited to -6°. Greater or lesser amountsof hyperextension can be permitted as desired.

In addition to carrying convex bearing surface 20 and cam means 43,femoral component 10 also includes fixation shank 16 which is adapted tobe implanted in the patient's femur using standard surgical techniques.Similarly, in addition to concave bearing surface 34 and cam means 41,tibial plateau component 12 also includes depending shaft 54 and thrustbearing surface 52. As shown in FIG. 2, depending shaft 54 canoptionally include metal reinforcing rod 74.

In the assembled joint, bearing surface 52 on the bottom of tibialplateau component 12 mates with bearing surface 58, i.e., the topsurface of tibial sleeve 14, and depending shaft 54 is received inaperture 56 formed in the body of the tibial sleeve. As fully describedin U.S. Pat. Nos. 4,219,893 and 4,301,553, referred to above, thepertinent portions of which are incorporated herein by reference, thisarrangement of these components allows tibial sleeve 14 to rotate withrespect to tibial plateau component 12 as the femur and tibia move froma position of full extension to a position of flexion. This rotation ofthe tibia about its longitudinal axis during flexion is normally in therange of 10°-15°.

Tibial sleeve component 14 is designed to be implanted in the upperportion of the tibia. Various approaches can be employed for thisimplantation. One such approach is that described in PCT PatentPublication No. W085/03426, entitled "Apparatus for Affixing aProsthesis to Bone," which is assigned to the same assignee as thisapplication. Briefly, this technique involves providing tibial sleeve 14with an outer surface 60 which has been contoured to mate with a portionof the inner surface of the hard bone at the upper end of the tibia. Inaddition to being anatomically contoured, the surface is also providedwith a geometry 62 designed to transform wedging shear loading tocompression loading in the tibial bone. A further discussion of thetechnique can be found in the above-referenced patent publication, thepertinent portions of which are incorporated herein by reference.

In addition to engaging tibial component 13, femoral component 10 alsoengages patellar prosthesis 80. For this purpose, the anterior surfaceof the femoral component includes concave recess 82 and the distalsurface includes track 84 which intersects the recess. Track 84 iscomposed of rails 88, each of which has a cross-sectional contour in adirection transverse to the longitudinal axis of the rail which is (1)either a straight line or a concave curve, (2) constant along the lengthof the rail, and (3) matches the contour of the surface of recess 82 atthe intersection between the rail and the recess.

The advantage of constructing rails 88 in accordance with the inventionis illustrated in FIGS. 12 and 13. FIG. 12 shows the construction usedin the prior art wherein the portion of the distal surface of femoralprosthesis 92 which engaged patellar prosthesis 80 is convexly shaped.As shown in this figure, the two prostheses only make point contact atpoints 90. Such point contact leads to high wear rates for the patellarprosthesis. Also, in such prior art prostheses, distinct slope changesexist at the intersection between the convex surface and the concaverecess formed in the prosthesis' anterior surface for receiving thepatellar prosthesis.

In contrast to FIG. 12, as shown in FIG. 13, when rails 88 are given theconfiguration of the present invention, line contact along curves 94 isachieved between the patellar and femoral prostheses. As shown in FIG.4, to achieve this line contact for a spherically-domed patellar button,curves 94 are portions of a circle having the same radius of curvatureR₅ as the domed surface of the patellar prosthesis. For typicalprosthesis dimensions, each of curves 94 can have a length, whichcorresponds to the width of the rail, on the order of 5-6 millimeters.

The line contact between the patellar prosthesis and the distal surfaceof the femoral prosthesis results in substantially reduced wear rates incomparison to those achieved with point contact. Also, as can be seenin, for example, FIG. 1, the concave contour of rails 88 results in acompletely smooth transition between the rails and concave recess 82.

In the case of a conical patellar button, rails 88 are portions of astraight line instead of being concave. In such a case, recess 82 wouldpreferably be V-shaped, i.e., composed of two inwardly sloping planes,so as to produce a smooth transition between the rails and the recess.

A patellar prosthesis 96 for use with the present invention whichachieves even greater contact with the distal surface of the femoralprosthesis is shown in FIG. 18. The anterior surface of this prosthesisincludes peg 98 for attaching the prosthesis to the posterior surface ofthe patient's patella. The posterior surface of the prosthesis, whichengages the femoral prosthesis in the assembled joint, has a saddleshape.

As shown in FIG. 18, the saddle has a radius of curvature R₅ in themedial-lateral direction. This curvature matches the medial-lateralradius of curvature of rails 88 (see FIGS. 4 and 13), and thus contactlike that achieved for a spherically-domed patellar prosthesis isachieved in this direction. Along lines 100 and 102, the saddle hasradii of curvature of R₂ and R₂ ', respectively. As can be seen in FIG.6, R₂ is the radius of curvature of the outer edge of rails 88, while R₂' is the radius of curvature of the inner edge of the rail. Accordingly,patellar prosthesis 96 will make surface contact throughout areas 104with rails 88 of the femoral prosthesis when the patellar button is incontact with the R₂ section of track 84, which condition exists duringthe high contact forces created by flexion and weight bearing.

This enhanced contact is illustrated in the sectional views of FIGS. 19and 20. As shown in FIG. 20, through the use of concave rails 88, aspherically-domed button is able to achieve line contact along line 138.However, as shown in FIG. 19, by using concave rails and a button havingthe saddle contour of FIG. 18, contact is achieved throughout area 140.

FIGS. 14-20 illustrate a constrained ("hinged") artificial knee jointemploying concave rails 88 for engaging either spherically-domedpatellar prosthesis 80 or saddle-shaped patellar prosthesis 96. For thisprosthesis, tibial plateau 12 carries hinge post 106. For this purpose,the tibial plateau includes slot 120 having side walls 122. The hingepost is mounted to the tibial plateau by snapping flanges 110 underbeads 108 formed in side walls 122.

Hinge post 106 includes hole 112 for receiving two flanged bearings 114,one from each side. The joint is assembled by slipping femoral component10 over the hinge post to bring bearing surfaces 130 on the femoralcomponent into contact with bearing surfaces 132 on the tibial plateau.Hinge pin 116 is then slid through holes 126 in the femoral componentand through bearings 114 to assemble the hinge. Hinge pin 116 is held inplace by means of snap rings 118 which are received in grooves 128.

As in the joint of FIGS. 1-11, the joint of FIGS. 14-20 has largebearing surfaces which extend across essentially the full width of thejoint. Specifically, the bearing surfaces associated with the femoralside of the joint comprise surfaces 130 and lower outer surface 134 ofhinge pin 116. The corresponding concentric bearings surfaces on thetibial side comprise surfaces 132 and the lower inner surface ofbearings 114.

Hinge pin 116 and femoral component 10 are made from materials havingsimilar wear characteristics, e.g., from titanium or cobalt chromiumalloys. Similarly, bearings 114 and tibial plateau 12 are made frommaterials having similar wear characteristics, e.g., from ultra-highmolecular weight polyethylene. In this way, the concentricity of thebearing surfaces is maintained as those surfaces wear since the wearablecomponents, i.e., the plastic components, are all associated with oneside of the joint, e.g., the tibial side, and are all surfaces ofrevolution about the same center line, while the non-wearablecomponents, i.e., the metal components, are all associated with theother side of the joint, e.g., the femoral side, and are surfaces ofrevolution about the same center line as the plastic components.Accordingly, as the plastic components wear, the common center line willsimply shift, e.g., move towards the tibia, with each of the metal andplastic bearing surfaces remaining concentric to its mating surface.

Bearing surfaces 130 are located further from the midplane of the jointthan bearing surfaces typically used in hinged joints. This changeallows rails 88 to be moved far enough apart to provide a stableengagement with the patellar prosthesis. During articulation of thejoint, rails 88 ride inside of walls 122 and are received in slot 120.The outside surface of the femoral component 10 includes recesses 142which engage protuberances 144 at the limit of the extension of thejoint.

Femoral component 10, tibial component 13 and patellar components 80 and96 can be made out of a variety of biologically compatible, surgicallyimplantable materials. For example, a cobalt-chromium-molybdenum alloy,such as that described in ASTM F75, can be used for femoral component10, a titanium-aluminum-vanadium alloy, such as that described in ASTMF136 can be used for tibial sleeve 14, and ultra-high molecular weightpolyethylene (UHMWPE) can be used for tibial plateau component 12,bearings 114 and the patellar prostheses. Similarly, hinge post 106,hinge pin 116, and snap rings 118 can be made of cobalt-chrome ortitanium alloys. Other types and combinations of materials appropriatefor use in the artificial joint of the present invention will be evidentto persons skilled in the art.

Although specific embodiments of the invention have been described andillustrated, it is to be understood that modifications can be madewithout departing from the invention's spirit and scope. For example,bearing surfaces have configurations other than those shown herein canbe used in the practice of the invention. Similarly, a variety of cammeans other than those described can be used to permit rolling of thebearing surfaces relative to each other during the minor segment of thejoint's motion.

What is claimed is:
 1. A prosthetic joint for providing flexion motionbetween two bones comprising:(a) a convex bearing component having afirst bearing area and a contiguous second bearing area, the secondbearing area being a surface of revolution about a first axis and havinga radius of curvature which is less than the radius of curvature of thefirst bearing area; and (b) a concave bearing component having a thirdbearing area for engagement during flexion with the first bearing areaand a contiguous fourth bearing area for engagement with the secondbearing area, the fourth bearing area being a surface of revolutionabout a second axis and having a radius of curvature which is less thanthe radius of curvature of the third bearing area;wherein: the radius ofcurvature of the second bearing area is substantially the same as theradius of curvature of the fourth bearing area so that the second andfourth bearing areas may be in congruent engagement with one another;and the flexion axis of the joint is parallel to the first and secondaxes when the second and fourth bearing areas are in engagement.
 2. Theprosthetic joint of claim 1 wherein the convex bearing componentcomprises the femoral component of an artificial knee joint and whereinsaid convex bearing component includes a body having an outer surface,said outer surface having first and second portions, the first portionbeing located anteriorly and the second portion being located distallywhen the prosthesis is implanted in the femur, the first portion of theouter surface including a surface recess for engagement with a patellarprosthesis, the second portion of the outer surface including a trackfor engagement with said patellar prosthesis, the track being composedof two rails, the cross-sectional contour of the surface of each of therails in a direction transverse to the longitudinal axis of the railbeing selected from the group consisting of a straight line and aconcave curve, said cross-sectional contour being constant along thelength of the rail, said rails extending from said surface recess andthe cross-sectional contour of the surface of each of said rails beingthe cross-sectional contour of the surface of the surface recess at theintersection between the rail and the surface recess so that the surfaceof each rail flows continuously into the surface of the surface recess.3. The prosthetic joint of claim 2 wherein the cross-sectional contourof each of the rails is concave and is a portion of a circle.
 4. Theprosthetic joint of claim 2 further including a patellar prosthesiswherein the patellar prosthesis makes contact with each of the railsalong a line which is transverse to the longitudinal axis of the rail.5. The prosthetic joint of claim 3 further including a patellarprosthesis wherein the patellar prosthesis has a saddle-shaped surfaceand wherein, for at least a portion of the track, the patellarprosthesis makes contact with each of the rails over an area extendingalong the length of the rail.
 6. The prosthetic joint of claim 1 whereinthe convex bearing component comprises the femoral component of anartificial knee joint and wherein said convex bearing component includesa body having an outer surface a portion of which is located distallywhen the prosthesis is implanted in the femur, said portion including atrack for engagement with a patellar prosthesis, said track beingcomposed of two rails, the cross-sectional contour of the surface ofeach of the rails in a direction transverse to the longitudinal axis ofthe rail being (a) selected from the group consisting of a straight lineand a concave curve, and (b) constant along the length of the rail. 7.The prosthetic joint of claim 6 wherein the cross-sectional contour ofeach of the rails is concave and is a portion of a circle.
 8. Theprosthetic joint of claim 6 further including a patellar prosthesiswherein the patellar prosthesis makes contact with each of the railsalong a line which is transverse to the longitudinal axis of the rail.9. The prosthetic joint of claim 7 further including a patellarprosthesis wherein the patellar prosthesis has a saddle-shaped surfaceand wherein, for at least a portion of the track, the patellarprosthesis makes contact with each of the rails over an area extendingalong the length of the rail.
 10. The prosthetic joint of claim 1wherein the contact area between the convex and concave bearingcomponents is greater when the second and fourth bearing areas are inengagement than when the first and third bearing areas are inengagement.
 11. A prosthetic knee joint comprising:(a) a femoralcomponent having a flexion bearing surface which includes a firstbearing area and a contiguous second bearing area, the second bearingarea being a surface of revolution about a first axis and having aradius of curvature which is less than the radius of curvature of thefirst bearing area; and (b) a tibial component having a flexion bearingsurface which includes a third bearing area for engagement duringflexion with the first bearing area and a contiguous fourth bearing areafor engagement with the second bearing area, the fourth bearing areabeing a surface of revolution about a second axis and having a radius ofcurvature which is less than the radius of curvature of the thirdbearing area;wherein: the radius of curvature of the second bearing areaof the femoral component is substantially the same as the radius ofcurvature of the fourth bearing area of the tibial component so that thesurface of revolution of the fourth bearing area of the tibial componentmay be in congruent engagement with the surface of revolution of thesecond bearing area of the femoral component; and the flexion axis ofthe joint is parallel to the first and second axes when the second andfourth bearing areas are in engagement.
 12. The prosthetic knee joint ofclaim 11 wherein the contact area between the flexion bearing surfacesof the femoral and tibial components is greater when the second andfourth bearing areas are in engagement than when the first and thirdbearing areas are in engagement.